1. Field of the Invention
This invention relates to exposure control apparatus for photographic cameras and more particularly to such exposure control apparatus including light responsive lag compensation.
2. Discussion Relative to the Prior Art
Many photographic cameras include automatic exposure control apparatus adapted to terminate exposure, for example, by quenching an electronic flash tube and/or by closing a shutter, in response to an exposure termination signal. The exposure termination signal is generated by a light responsive timing circuit that produces the signal as a function of a time integral of scene light intensity. All such exposure control apparatus exhibit a characteristic time delay from the time the exposure termination signal is generated to the time that actual exposure is terminated. This time delay, is commonly called the "lag time." If the lag time is associated with the quenching of electronic flash, it will be herein referred to as "quench lag," and if associated with the closing of a shutter, it will be referred to as "shutter lag."
The exposure that occurs during a lag time can be a significant portion of the total exposure, and unless the lag is effectively accounted for by the exposure control apparatus, an appreciable exposure error might result.
One way of accounting for lag --represented by U.S. Pat. No. 3,200,723 for shutter lag compensation and U.S. Pat. No. 3,519,879 for quench lag compensation--is to continuously bias an integrated photocell response by a function of the instantaneous light intensity-I(t). Such lag compensation arrangements require an external mode switch to deactivate the lag compensation when an exposure is made in ambient light, and are not adapted for exposures in the fill-flash mode.
The referenced copending patent application discloses an exposure control apparatus including general purpose lag compensation. The apparatus includes a lead time for lag compensation in ambient light and means responsive to a change in light intensity for lag compensation with flash or fill-flash illumination. Such exposure control apparatus accomplishes lag compensation with the additional advantage that external mode switching is not required to change from ambient mode to a flash mode of exposure. The function of such apparatus can be described as solving, for T, an exposure equation of the form: ##EQU1## where: E is a predetermined constant representing the required exposure,
I(t) is the intensity of scene light as a function of time, PA1 C is a predetermined constant, PA1 O is the time at which exposure measurement is commenced, and PA1 T is the time required to achieve E. PA1 I(t) is the intensity of scene light at the time that the exposure termination signal is generated.
Or upon integrating, equation 1 becomes: ##STR1## where: I(O) is the intensity of scene light at the instant exposure measurement is commenced, and
The lag compensation arrangements discussed above generally improve flash exposure for the intermediate to distant flash range; however, there is still observed a substantial flash error for extremely close-up flash exposures. One approach that has been suggested for providing additional compensation for lag during extremely close-up flash photography is to provide a time-variable bias on the trip point of a level sensitive trigger circuit. The time-variable bias is caused to decrease relatively rapidly, thereby affecting only extremely close-up exposures that "time out" in a relatively short time. Extremely close-up, hence, short, exposures receive a correction inversely proportional to their time duration. Such a scheme for applying a time-variable bias to the trip point of a trigger circuit is shown in U.S. Pat. No. 3,737,721 where it is disclosed in the context of compensation for electronic flash quench lag. If used to compensate for shutter lag, this scheme would necessarily have to be disabled during ambient mode photography, thereby requiring the addition of external mode switches.
For the reasons outlined above, it is desirable to have an improved exposure control apparatus that compensates for the effect of lag during extremely close-up flash photography and also works in the ambient mode of exposure without the requirement for external mode switches.